Package for light-triggered thyristor

ABSTRACT

An hermetically sealed package for a light-triggered thyristor. The thyristor is mounted within the cavity of an insulating body. Cathode and anode pole pieces are mounted on opposite sides of and electrically coupled with the thyristor. Annular flanges are provided to form hermetical seals between the body and pole pieces. A light pipe is mounted in a radially extending slot formed through a face of one pole piece. An inner end of the light pipe is optically coupled with a light-sensitive region of the thyristor. The outer end of the light pipe is mounted within a metal sleeve which radially projects through an opening formed in a side of the body. An hermetical seal is formed between the light pipe and inner end of the sleeve by means of a glass frit in one embodiment, or by means of solder bonded between the sleeve and a metallized region formed about the light pipe in another embodiment. Another hermetical seal is formed by solder which bonds between the metal sleeve and a metallized region formed in the body about the opening. A connector fitting is mounted on an outer end of the sleeve for seating the end of a fiber optic cable which extends from a triggering light source.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 800,706 filed May26, 1977, now U.S. Pat. No. 4,131,905, entitled Package forLight-Triggered Thyristor.

BACKGROUND OF THE INVENTION

This invention relates in general to semiconductor devices and inparticular relates to a package for thyristors of the type used in powercircuits.

Thyristors are employed for various purposes in electrical power andcontrol circuits. An example is the use of thyristors as high voltagedistribution equipment. In such applications control problems arise dueto factors such as noise signal pickup, also, high voltage potentialsacross the control leads used to trigger the thyristors must be avoidedby expensive means such as individual transformers.

It would be desirable to employ a light-triggered thyristor in powercircuits of the foregoing nature so as to isolate the control signalfrom the power signal and thereby eliminate noise pickup and controlproblems. However, it has heretofore not been feasible to employlight-triggered thyristors in power circuits because of the necessityfor hermetically sealing such thyristors. The pole faces of powersemiconductor devices require massive thermal and electrical contact tohandle the relatively large currents, and it has not been feasible tointroduce a triggering light signal to a light-sensitive region of oneof these pole faces while maintaining hermeticity of the package. Thereis further the requirement of providing a suitable external connectionbetween the package and the light source, such as a fiber optic cable.Previously the differences in coefficients of thermal expansion betweenglass light pipes and other elements of the package have precluded apractical hermetically sealed thyristor package for power circuits.

OBJECTS AND SUMMARY OF THE INVENTION

It is a general object of the invention to provide a new and improvedpackage for light-triggered semiconductor devices of the type used inpower circuits.

Another object is to provide a package of the type described whichintroduces a triggering light signal to a light-sensitive region of thesemiconductor device while maintaining hermetical sealing of thepackage.

Another object is to provide an hermetically sealed package for alight-triggered thyristor for use in circuits where a control signal isto be electrically isolated from the power signal.

Another object is to provide a light-triggered thyristor package whichpreserves hermeticity of the package by providing hermetical seals atthe opening through which the light pipe emerges from the package forcoupling with a triggering light source.

The invention in summary includes an insulating body having a cavity inwhich a light-triggered semiconductor device is mounted. Cathode andanode pole pieces are mounted on opposite sides of the device, andhermetical seals are formed between the pole pieces and respective endsof the body. A light pipe is mounted within a radially extending slotformed in a face of one pole piece. An inner end of the light pipe isoptically coupled with a light-sensitive region of the semiconductordevice. The outer end of the light pipe carries a metal sleeve whichextends through an opening formed in the body. The sleeve is formed of amaterial having a coefficient of thermal expansion compatible with thatof the light pipe and with the body. An hermetical seal is formedbetween an inner end of the sleeve and the light pipe and anotherhermetical seal is formed between the outer surface of the sleeve and ametallized region of the body about the opening. A connector fitting ismounted on the outer end of the sleeve for holding the end of a fiberoptic cable which is connected with a triggering light source.

The foregoing and additional objects and features of the invention willappear from the following description in which the preferred embodimentshave been set forth in detail in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a thyristor package according to oneembodiment of the invention.

FIG. 2 is a fragmentary view to an enlarged scale showing the hermeticalsealing arrangement around the light pipe of the package shown in FIG.1.

FIG. 3 is a fragmentary view to an enlarged scale showing the hermeticalsealing arrangement around the light pipe in the thyristor package ofanother embodiment.

FIG. 4 is a cross-sectional view taken along the line 4--4 of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings FIG. 1 illustrates an embodiment of the inventionproviding a package 10 for hermetically sealing a light-triggeredsemiconductor device 12, specifically a light-triggered thyristor. Thethyristor package of the illustrated embodiment is specially adapted foruse in circuits where it is desired to electronically isolate thecontrol signal from the power signal.

Package 10 includes an annular electrical insulating body 14 comprisedof a suitable ceramic material, preferably a high alumina ceramic. Theinner wall 16 of the body is cylindrical to define a cavity 18, and theouter wall is formed with circular ribs 20 for increasing the surfaceresistance path, as is well known. A suitable inert gas such as nitrogenis filled within cavity 18 when the package is assembled andhermetically sealed in the manner described below.

Thyristor 12 is positioned within the body between a pair of pole pieces22, 24. The pole pieces preferably are of nickel-plated copper and theyprovide the anode and cathode contacts. The thyristor comprises asilicon wafer having a light-sensitive region 26 formed on its upperface. The thyristor is centered between the pole pieces by means of alocator ring 28. The ring is shaped with an intrusive flange 30 whichseats in a circular groove formed about the upper face of the lower polepiece. A circular disc 32 formed of a suitable electrical conductivematerial such as silver is mounted between the thyristor and the upperface of pole piece 22. Holes 33, 34 are partially drilled into the outerfaces of the pole pieces to provide seats for the positioning pins ofsuitable copper heat sinks, which are conventional and thus need not beshown.

Lower pole piece 22 is mounted within the lower end of body 14 by meansof an annular cap 36 formed of a suitable metal such as copper.Hermetical seals are formed at the junctures between cap 36 and thelower pole piece 22 and between the cap and body 14 by brazing with asuitable material such as silver-copper alloy. Prior to brazing, aregion 37 of the ceramic body adjacent the cap is metallized toestablish a bond with the brazing material. The metallization can becarried out by firing a Mo-Mn paste at high temperature on the desiredregion of the ceramic. Molecules of the metal in the paste diffuse intoan outer layer of the ceramic so that the diffused molecules can bondwith the braze alloy.

Upper pole piece 24 is mounted within the upper end of the body by meansof a pair of flanges 38, 40 formed of a metal such as copper. Lowerflange 38 is hermetically sealed to the body 14 by brazing with asuitable material such as a silver-copper alloy. Prior to brazing, aregion 41 of the ceramic body adjacent the flange is metallized by theprocedure explained above. Upper flange 40 is cold welded about thecircumference of the lower flange, and the inner rim of the upper flangeis bonded about pole piece 24 by brazing with a silver-copper alloy.

A narrow slot 42 is formed diametrically across the lower face of upperpole piece 24. A light pipe 44 is mounted within one side of this slot.The light pipe is formed of a glass rod 45 bonded within a concentricglass jacket 46. The inner end of the light pipe is shaped with a bightor arcuate portion 47 terminating at a flat end 48 which abuts thelight-sensitive region of the thyristor at substantially a right angle.A suitable adhesive 50 such as clear epoxy is bonded between flat end 48and the thyristor face for improving optical coupling and to hold therod in proper position.

Outer end 52 of the light pipe extends radially through an opening 54formed in a side of body 14. An elongate hollow sleeve 56 is mountedabout the outer end of the light pipe concentrically within the opening.Sleeve 56 is formed of a material having a coefficient of thermalexpansion compatible with the coefficients of thermal expansin of theglass light pipe and glass frit as well as the ceramic material formingthe body. Preferably the sleeve is formed of KOVAR, a trademark ofWestinghouse Electric Corporation for an iron-nickel-cobalt alloy, or itcan be formed of Carpenter 426, a trademark of Carpenter Steel Co. for a42% nickel-iron steel alloy. The KOVAR alloy has a coefficient ofthermal expansion in the range of 4.6-5.2×10⁻⁶ /°C., while the Carpenter426 has a coefficient of 7.4×10⁻⁶ /°C. These alloys are thus compatiblewith the preferred materials of the light pipe and body. For example, ina typical light pipe the inner glass rod has a coefficient of 9.1×10⁻⁶/°C. and the outer glass jacket has a coefficient of 5.15×10⁻⁶ /°C. Alsoby way of example a high alumina ceramic suitable for the insulatingbody has a coefficient of thermal expansion of 7.8×10⁻⁶ /°C.

An hermetical seal is formed between sleeve 56 and the light pipe 44 byfusing a glass frit 58 between the inner end of the sleeve and the glassjacket 46. Preferably the frit material is Corning #7583, a trademark ofthe Corning Glass Company for a soft glass which flows and bonds at atemperature less than 500° C.

An hermetical seal is also formed by solder material 60 bonded about theouter surface of the sleeve and the body opening 54. Preferably theouter surface of the sleeve is initially plated with a metal such asnickel. A region 62 of the ceramic material about the opening is alsoinitially metallized by the procedure described above for metallizingthe regions 37 and 41. The solder seal material 60 preferably is alead-tin solder (e.g. 60% Sn-40% Pb, M.P. 183° C.) which bonds both withthe metallized region of the ceramic and the nickel plating about thesleeve.

A connector fitting or ferrule 64 formed of a suitable rigid materialsuch as metal or hard plastic is mounted about the outwardly projectingend of sleeve 56. A suitable adhesive material 66 such as epoxy isapplied to bond the ferrule to the ceramic body and to the sleeve. Theouter end of the ferrule extends beyond the end of light pipe 44 toprovide a socket 68 for holding the end of a flexible fiber optic cable70. The cable extends from a remote triggering light source, not shown.

FIGS. 3 and 4 illustrate in partial view an embodiment of the inventionwhich provides different means for forming an hermetical seal about thelight pipe where it emerges from the package. In this embodiment thepackage includes the similar elements for the package 10 of the firstembodiment including a light-triggered thyristor mounted between anodeand cathode contacts within the cylindrical cavity of a ceramicinsulating body 72. A light pipe 74 comprising a glass rod 76 bondedwithin a concentric glass jacket 78 is optically coupled at its innerright angle bent end with the light-sensitive region of the thyristor.The light pipe extends outwardly through a radial slot formed in onepole piece and thence through a circular opening 80 formed in a side ofthe insulating body.

An hermetical seal is established between the light pipe and insulatingbody by means which includes forming an annular metallized region 82 inthe outer layer of the end of the light pipe which emerges from thebody. The metallized region can be formed by applying a coat of asuitable metal-glass frit, e.g. Mica Silver Type II sold by TranseneCo., about the outer surface of the light pipe, drying the coating at atemperature of 100° C. for fifteen minutes, and then firing the lightpipe with the coating in a furnace at a temperature of 450° C. for fiveminutes so that the metal molecules diffuse into the glass of the pipe.A layer 84 of a metal such as copper is then plated about the metallizedportion of the light pipe. The copper layer can be formed by applyingthe suitable CuSO₄ -H₂ SO₄ plating solution to a thickness of 0.001".The copper plated light pipe is then hand burnished with fine steel woolto remove any copper nodules or whiskers. A layer of solder is thenapplied about the copper plating by first dipping the plated end into asuitable flux and then dipping the end into a molten solder such as 96%Sn-4% Ag solder (M.P. 221° C.). A metal tubing 86, preferably nickeltubing, having an inner diameter 88 commensurate with the outer diameterof the copper plated end of the light pipe is then fluxed and dipped insolder of the type described above. The light pipe and sleeve are heatedtogether, and the light pipe is then pushed into the sleeve until theends are approximately flush. Upon cooling, the resulting solder layer90 forms an hermetical seal between the copper plated light pipe andsleeve. The ends of the light pipe and sleeve are then cut to size andpolished.

An hermetical seal is formed between the nickel sleeve and the ceramicbody by first forming an annular metallized region 92 about body opening80 in the manner described for the embodiment of FIG. 1. The nickelsleeve encased end of the light pipe is inserted through the bodyopening where it can be loosely supported by suitable means such as aTeflon plug, not shown, while the package is being heated in an oven forcuring the epoxy which bonds the inner end of the light pipe to thelight-sensitive region of the thyristor. After the epoxy is cured thepackage is transferred to an oven held at a temperature on the order of195°-200° C. A piece of solder wire, such as 60% Sn-40% Pb, M.P. 183°C., is dipped into a suitable activated rosin flux and inserted intobody opening 80 so that the solder completely fills the space betweenthe nickel sleeve and opening. Upon cooling, the resulting solder layer94 forms an hermetical seal. A connector fitting for attaching the endof a fiber optic cable, not shown, can then be secured to the outwardlyprojecting end of the sleeve and light pipe in the manner described forthe embodiment of FIG. 1.

The use and operation of the invention is as follows. The thyristorpackage 10 can be assembled and connected in a power circuit to providean efficient transfer of an externally generated optical signal to thelight-sensitive region of the semiconductor device. At the same time thedevice is maintained in an hermetically sealed inert gas atmospherewhile also preserving a relatively large surface contact between thepole pieces and the device to provide good thermal conduction and tocarry large currents. During fabrication of the package the step ofhermetically sealing the package is completed by cold welding operationsto be performed with existing equipment, components of which wouldotherwise interfere with the connector. Because an hermetical seal hasalready been established between the light pipe, sleeve and ceramicbody, the adhesive bond holding the connector fitting need not behermetic. The package is installed in the desired electrical circuitwith the end of the fiber optic cable inserted in fitting 64. An opticalsignal from the remote source carries through the cable and along lightpipe 44 for triggering the thyristor.

While the foregoing embodiments are at present considered to bepreferred it is understood that numerous variations and modificationsmay be made therein by those skilled in the art and it is intended tocover in the appended claims all such variations and modifications asfall within the true spirit and scope of the invention.

What is claimed is:
 1. An hermetically sealed package for a light-triggered semiconductor device, comprising the combination of an electrical insulating body having a central cavity, a light-triggered semiconductor device within the cavity, first and second pole pieces positioned within opposite ends of the body and having respective inner pole faces juxtaposed with and electrically coupled to opposite sides of the semiconductor device, means for forming hermetical seals about the first and second pole pieces and respective ends of the body, a glass light pipe having an inner end optically coupled with a light-sensitive region of the semiconductor device and an outer end extending through the body, said outer end adapted to be optically coupled with a light source for triggering said device, a hollow metal sleeve mounted about a portion of the outer end of the light pipe, means for forming an hermetical seal between the light pipe and the sleeve comprising means forming a metallized region about the light pipe along the portion about which the sleeve is mounted, and solder bonded between the metallized region of the light pipe and the sleeve.
 2. A package as in claim 1 in which the means forming the seal further includes a layer of copper plating about the metallized region of the light pipe with the solder bonded both to the copper plating and to the sleeve.
 3. A package as in claim 1 in which the sleeve is formed of nickel.
 4. An hermetically sealed thyristor package including the combination of an annular electrical insulating body, a pair of electrical contacts coaxially mounted within the body in spaced-apart relationship, means forming hermetical seals between the contacts and respective ends of the body, a light-triggered thyristor mounted coaxially between and electrically coupled with the contacts, means forming a passageway through a side of one of the contacts, a light pipe having a radially directed outer end extending along the passageway and through the body, the light pipe having an arcuate inner end abutting a light-sensitive region of the thyristor, a metal sleeve mounted coaxially about the outer end of the light pipe, means forming an hermetical seal between the sleeve and the light pipe comprising a region of metal molecules diffused about the portion of the light pipe on which the sleeve is mounted, a layer of metal plating bonded about the region of diffused metal molecules, and solder bonded between the metal layer and the sleeve, means forming an hermetical seal between the sleeve and the body, a connector fitting mounted about a portion of the sleeve which projects outwardly from the body, said fitting having a socket-shaped end for mounting the end of a fiber optic cable coupled with a triggering light source.
 5. A thyristor package as in claim 4 which includes means forming a radially extending opening through the body with the sleeve and outer end of the light pipe projecting coaxially through the opening, and the means forming the seal between the sleeve and body includes means forming a zone of metallized material in the body about the opening together with solder seal means in the opening for forming a bond with the metallized material and with the outer surface of the sleeve. 